A signal is transmitted over a long distance in the form of light without being converted into electrical signal in an optical network. A device amplifying the signal in the form of light is an optical amplifier. The transmission distance is increased, because the optical amplifier amplifies the power of the signal. On the other hand, a noise is generated at the optical amplifier. Heavy use of the optical amplifier may cause cost increase. It is therefore preferable that the number of optical amplifier is optimized. Design of optical amplifier location in network design is to determine where the optical amplifier is located in the network. Japanese Patent Application Publication Nos. 2008-507223, 2006-135788 and 2004-48477 disclose the design of optical amplifier location.